10 10.1 Recent Advances in the Asymmetric Hydrogenation of Imines
Chiral aromatic amines are extremely interesting products because of their applications in the pharmaceutical, agrochemical and fi ne chemical industries. The enantioselective hydrogenation of C = N double bonds using chiral transition metal complexes as catalysts is one of the most useful methods for preparing optically active amines [1] . This reaction has drawbacks, however: coordination can take place through both the nitrogen donor atom and the double bond, and the substrate and catalyst intermediates are unstable under catalytic conditions. The C = N substrates, for instance, are often sensitive to hydrolysis. Homogeneous catalysts can complex with both the imine and the amine product or can be deactivated by the formation of trimers or oligomers. In consequence, catalytic activity is often low. Furthermore, in the case of the acyclic imines one of the major problems for achieving high enantiomeric excess is the equilibrium between the E and Z isomer of the imine, which makes it diffi cult for the catalyst to convert all stereoisomers in selective manner. Many ruthenium -, rhodium -and iridium -based catalytic systems are excellent in hydrogenating functionalized olefi ns and ketones but are much less effi cient with imine substrates.
In spite of these problems, an Ir -catalyzed imine hydrogenation is the key step in the largest scale enantioselective catalytic process used in industry, which is also one of the fastest homogeneous catalytic systems: the industrial production of the chiral herbicide ( S ) -metolachlor [ N -(1 โฒ -methyl -2 โฒ -methoxyethyl) -N -chloroacetyl -2ethyl -6 -methylaniline] in amounts greater than 10 000 tonnes per annum at about 80% ee [2] . The catalyst is a combination of [Ir(COD)Cl] 2 ( COD = 1,5cyclooctadiene ), the chiral ferrocenyl diphosphine Xyliphos, tetrabutylammonium iodide ( TBAI ) and sulfuric acid. 2 -Methyl -6 -ethylphenyl -1 โฒ -methyl -2 โฒ -methoxyethylimine ( MEA(monoethanolamine) -imine ) is hydrogenated under 80 bar (8 MPa) hydrogen pressure at 323 K and at a substrate/catalyst ratio exceeding 10 6 to yield MEA -amine in 79% ee and with an initial turnover frequency that is said to exceed 1.8 ร 10 6 h -1 (Scheme 10.1 ). This process has been extensively studied and